1. Field of the Invention
This invention relates to pneumatic tires for motorcycles, and more particularly to a pneumatic tire for a front wheel of a motorcycle capable of preventing the occurrence of shimmy in the high-speed running to improve a steering stability, particularly straight running stability.
2. Description of Related Art
In the pneumatic tire particularly mounted onto a front wheel of a motorcycle, the steering performances such as handling and the like are considered to be important, so that it is usual to adopt a belt comprised of at least two belt layers, cords of which belt layers being crossed with each other, or so-called cross structure belt up to the present time.
With the recent advancement of weight reduction and higher performances in a vehicle, a micro-oscillation of a handle or so-called shimmy phenomenon is actualized in the motorcycle. Particularly, it is strongly demanded to prevent the occurrence of shimmy at a high frequency zone. However, the tire having the above cross structure belt is difficult to flexibly absorb the pushing up of micro-irregularities on the road to the tread surface of the tire and the micro-oscillation of the handle resulted therefrom.
And also, the motorcycle is more important to ensure the stability of the vehicle body in the high speed or extra-high speed running. In the above pneumatic tire for the motorcycle, however, the pushing quantity of a central portion of the tread becomes large with the increase of the speed to decrease the ground contacting area of the tread, which causes the lowering of the stability in the high speed running.
On the other hand, a tire having a belt formed by spirally winding a cord having a high modulus of elasticity such as high-modulus polyester or rayon fiber cord, aromatic polyamide fiber cord or the like on the outer periphery of the carcass ply in a widthwise direction of the tire or so-called spiral structure belt is used for a rear wheel of the motorcycle. This tire has many characteristics that the outward pushing of the tread is prevented by hoop effect of the spiral structure belt even in the high speed running, and heat generation and rolling resistance are small owing to small movement of tread rubber, and the wear resistance is excellent, and the decrease of the ground contacting area with the rise of the speed is small. However, when the tire having the above spiral structure belt is used in the front wheel of the motorcycle taking count of steering performances such as steering stability and the like, since the cord in the belt extends substantially in the circumferential direction of the tire, the bending rigidity of the tread portion in the widthwise direction thereof is too low as compared with the cross structure belt, and hence the handle response, gripping force on road surface and the like are largely lacking and the steering performances are degraded. Therefore, the tire having the spiral structure belt is not used in the front wheel of the motorcycle.